Liquid crystal devices typically comprise a pair of opposed, spaced-apart translucent cell walls with liquid crystal (“LC”) material between them. The cell walls have transparent electrode patterns for applying fields to align the LC material.
LC materials are rod-like or lath-like molecules which have different optical properties along their long and short axes. The molecules exhibit some long range order so that locally they tend to adopt similar orientations to their neighbours. The local orientation of the long axes of the molecules is referred to as the director. When the director is orientated perpendicular to the plane of the cell walls, this is referred to as homeotropic alignment. Alignment of the director along the plane of the cell walls or at an angle to the plane of the cell walls is referred to respectively as planar homogeneous and tilted homogeneous alignment.
There are three types of LC materials: nematic, cholesteric (chiral nematic), and smectic. The present invention concerns devices using nematic LC materials, which may optionally be chiral or chirally doped.
Typical LC displays which employ nematic LC materials are monostable, application of an electric field causing the LC molecules to align in an “on” state, and removal of the electric field permitting the LC molecules to revert to a pre-determined “off” state. Examples of such monostable modes are twisted nematic (TN), supertwisted nematic (STN) and hybrid aligned nematic (HAN) modes. Each “on” pixel must be maintained above an electric field threshold, which can cause problems in the matrix addressing of complex displays. These problems can be overcome by driving each pixel by a thin film transistor (TFT), but manufacturing large area TFT arrays is difficult and adds to manufacturing costs.
A number of bistable LC devices have been proposed in which a nematic LC has more than one stable orientation of the director, and can be switched between two stable states when addressed by suitable waveforms.
U.S. Pat. No. 4,333,708 discloses a multistable LC device in which switching between stable configurations is by the movement of disclinations in response to electric fields.
In WO 91/11747 and WO 92/00546 it is proposed to provide a bistable surface by careful control of the thickness and evaporation of SiO coatings. A first stable planar orientation of the director could be obtained, and a second stable orientation in which the director is at an azimuthal angle (in the plane of the surface) of 90° to the first orientation in the plane of the surface, and tilted by around 30°.
It has been proposed, in GB 2,286,467, to achieve an azimuthal bistable surface by using a bigrating surface in which the director is planar to the surface and two surface orientations are stabilised by precise control of the dimensions of the grating.
In “Mechanically Bistable Liquid-Crystal Display Structures”, R N Thurston et al, IEEE Trans. on Elec. Devices, Vol. ED-27, No. 11, November 1980, there are described two bistable nematic LC modes which are called “vertical-horizontal” and “horizontal-horizontal”. In the vertical-horizontal mode, both cell walls are treated to give a roughly 45° tilt which permits the directors to be switched between two states in a plane which is perpendicular to the major surfaces of the device. In the horizontal-horizontal mode, the director is switchable between two angles in a plane parallel to the major surfaces of the device.
WO 97/14990 and WO 99/34251 describe the use of a monograting surface with a homeotropic local director, which has two stable states with different tilt angles within the same azimuthal plane. The homeotropic alignment is achieved by creating the monograting in a layer of material which causes spontaneous homeotropic orientation or, more practically, by coating the grating surface with a homeotropic inducing alignment agent such as lecithin. WO 01/40853 describes similar display technology in which small alignment areas having local homeotropic alignment are formed by a plurality of surface features such as grating areas, protrusions, or blind holes, and may be separated by areas of monostable alignment. Within each area there may be a graded variation so that the amount of scattering is dependent on amplitude of applied voltage, thus giving a greyscale effect.
We have now found that a bistable nematic LC device may be constructed using an alignment layer which induces substantially planar local alignment and which has an array of holes that are shaped so as to permit the director to adopt either of two tilt angles in substantially the same azimuthal direction. The cell can be switched between the two tilt states by an applied electric field to display information which can persist after the removal of the field.
The term “azimuthal direction” is used herein as follows. Let the walls of a cell lie in the x,y plane, so that the normal to the cell walls is the z axis. Two tilt angles in the same azimuthal direction means two different director orientations in the same x,z plane, where x is taken as the projection of the director onto the x,y plane.